Isoflavones, which are contained at high levels in soy foods, are known to be functional components effective for mitigating menopausal syndrome such as indefinite complaint; preventing osteoporosis; preventing hyperlipidemia and arteriosclerosis; preventing breast cancer and prostate cancer; etc. Recent studies have revealed that daidzein, a kind of isoflavone, is metabolized by the mediation of enterobacteria in the body to equol, which has stronger estrogen-like action and anti-oxidant action. Thus, equol has attracted attention as one of the important active ingredients exhibiting the aforementioned effects in the body.
Meanwhile, production of equol from daidzein does not equally occur in all humans, and equol-producing ability varies among individual humans. About 30 to 50% humans have equol-producing ability (Non-Patent Document 1). Therefore, extensive studies have been carried out to search for enterobacteria having equol-producing ability. Hitherto, the following microorganisms have been reported to have equol-producing ability: Bacteroides ovatus, Streptococcus intermedius, Streptococcus constellatus (Patent Document 1), Lactococcus garvieae (Patent Document 2), Slackia spp. strain TM-30, Bifidobacterium adolescentis strain TM-1, Bifidobacterium breve JCM 1273 (Patent Document 3), Proprionobacterium freudenreichii, Bifidobacterium lactis, Lactobacillus acidophilus, Lactococcus lactis, Enterococcus faecium, Lactobacillus casei, Lactobacillus salivarius (Patent Document 4), SNU-Julong 732 (Non-Patent Document 2), and Gram-positive bacterium do03 (Non-Patent Document 3).
However, since all the above-reported microorganisms exhibit a low conversion ability from daidzein as a substrate to equol (hereinafter may be referred to as “daidzein-to-equol conversion ability” or simply as “conversion-to-equol ability”), these microorganisms have not been employed in production of equol in the human body or in industrial production of equol.